1879.| Hillocks or Mound-Formations of San Diego, Cal. 571 
Next the plant perishes, and, deprived of its protection, the 
summit is reduced and the base widened as it is lowered (Fig. 3 4) 
till finally a remnant of the deposit has become so assimilated 
and compact as to constitute a more permanent summit (Fig. 3 ¢), 
or it has totally disappeared, leaving the summit at or below its 
original base (Fig. 3 d). 
Reasons for the appearance of these phenomena so exclu- 
sively on the Pacific slope and the arid plains of the West, are 
that the combination of causes resulting in their production there 
are seldom found elesewhere, to wit: the growth of shrubbery in 
compact clusters suitably distributed, with low and dense foliage, 
the presence of burrowing animals, the great susceptibility of the 
soil to wash and, I may add, the steady prevalence of winds from 
a single quarter, and the absence of forests which would other- 
wise influence winds and surface drainage. 
Note.—Since the foregoing was written it has been suggested 
to me by a gentleman whose opinions have much weight, that the 
wind exercises an influence in excavating the earth around and 
between the shrubs of which the mounds are a sequence. While 
there is no evidence of such action in this vicinity, the explana- 
tion doubtless holds good in sections of the country in which a 
loose or sandy soil prevails. The mounds of this vicinity are 
found almost exclusively on the upland which, when dry, is quite 
firm and is not perceptibly acted on by the wind, yet sweeping 
Over a considerable surface it gathers enough of soil, in time, to 
make large deposits about the shrubbery. Sandy soil is excep- 
tional, and is found usually only in the valleys which are com- 
paratively small in extent. In such situations the suitable vege- 
tation does not so commonly exist, there is more protection from 
the winds, and the rains, generally light, are so readily absorbed 
that no surface-wash takes place. 
It has also been suggested that pebbles and rocks form nuclei 
around which accumulations of soil remain and conduce to the 
VOL, XNI.—NO, IX. 39 
